Active Scanning Beam 3 Checking Delivery/Dosimetry

Transcription

1 Active Scanning Beam 3 Checking Delivery/Dosimetry C Algranati, J Salk, A Coray, A Lomax, E Pedroni, T Boeringer, S Lin, E Hug Center for Proton Radiation Therapy

2 Checking Delivery/Dosimetry Contents Absolute Dose Measurements for Protons Dosimetry Protocols Monitor Calibration Machine Specific Checks and Dosimetry Patient Specific Dosimetry Summary 2

3 Absolute dose measurements 1954: LBL, Berkley, USA 1957: GWI, Uppsala, Sweden 1961: HCL, Cambridge, Boston, USA 1967: JINR, Dubna, Russia 1969: ITEP, Moscow, Russia 1975: PINP, St. Petersburg, Russia 1983: PMRC, Tsubaka, Japan 1986: AAPM Report 16 (TG 20) Protocol for heavy charged particle therapy beam dosimetry 3

4 Absolute dose measurements ICRU report 59 (1998): First international standard for clinical proton dosimetry Thimble ionisation chamber dosimetry protocol 60 Co calibration in terms of air kerma or absorbed dose to water calibration coefficients Applies to protons only IAEA TRS 398 (2000): Ionisation chamber dosimetry protocol Based on absorbed dose to water calibration coefficients Code of practice for photon, electron, protons, and ions Comparison published by J. Medin, Phys.Med.Biol. 45 (2000): Differences in absolute dose of up to 2.6% for clinical proton beams ICRU 78 (2007) 4

5 Ionisation chamber dosimetry according to TRS 398 Ionisation chambers Both cylindrical and plane parallel chambers are recommended Plane-parallel chambers yield higher uncertainty in absolute D w, although better suited for relative dosimetry Cylindrical ionisation chambers recommended for SOBP lengths 2cm Plane-parallel chambers must be used for SOBP lengths < 2 cm Many commercial systems available (usually not explicitely specified as proton chamber) 5

6 Ionisation chamber dosimetry according to TRS 398 Basic formalism used in ionisation chamber dosimetry D w,q = M Q N D,w,Qo k Q,Qo M Q N D,w,Q o k Q,Qo Instrument reading at users beam quality Q, corrected for all influence quantities other than beam quality, e.g.: k elec calibration factor for electrometer k pt temperature and air pressure k S recombination losses Absorbed dose to water calibration coefficient for calibration beam quality Q 0 ( = Co-60 ) Beam quality factor to correct for effects of differences between calibration beam quality Q 0 and user beam quality Q This applies to any user beam quality (photons, electrons, protons, ions) 6

7 Ionisation chamber dosimetry according to TRS 398 Basic formalism used in ionisation chamber dosimetry for proton beams p Q 1 for protons p Q 1 for 60 Co 7

8 Reference dosimetry for scanned proton beams at PSI Faraday cup measurement: Determines number of incident particles in pencil beam Monitor calibration in terms of protons per MU Energy Protons / MU Pencil beam dose model: Predicts absolute dose per incident proton D(x,y,z) = T(z) G(x, z, σ x (z)) G(y, z, σ y (z)) 138 MeV 160 MeV 177 MeV Integral depth dose T(z): based on first principles (Bethe-Bloch stopping power formula) Corrections for nuclear interactions (proton flux attenuation) total nuclear cross sections of protons in hydrogen and oxygen Scheib S. Spot-Scanning mit Protonen: Experimentelle Resultate und Therapieplanung 1993, Diss. ETH Zürich Nr Pedroni E. et al. Experimental characterization and physical modelling of the dose distribution of scanned proton beams, Phys Med Biol Feb 7;50:

9 Reference dosimetry for scanned proton beams at PSI Faraday cup measurement Number of protons/mu Pencil beam dose model Predicts number of protons (or MU) needed to fill a 10x10x10 cm 3 box with homogeneous dose of 1.0 Gy Thimble chamber Apply scan to phantom Measure actual dose with certified thimble ionisation chamber following code of practice IAEA TR 398 Compare predicted and measured dose Correction factors for MU calculations 9

10 Reference dosimetry for scanned proton beams at PSI ±2% 10

11 Reference dosimetry for scanned proton beams at PSI METAS Develop primary standard for (scanned) proton beams based on water calorimetry with an uncertainty 1 % (D.Twerenbold) 11

12 Reference dosimetry for scanned proton beams at PSI Range Shifter Fast Degrader patient patient MU chamber MU chamber always sees constant proton energy during beam delivery MU calibration stays constant during beam delivery sees varying proton energies during beam delivery MU calibration changes during beam delivery 12

13 Reference dosimetry for scanned proton beams at PSI Energy dependent monitor calibration PSI HIT, Heidelberg Courtesy, A. Coray, PSI Oliver Jäkel et al., A calibration procedure for beam monitors in a scanned beam of heavy charged particles, Med. Phys. 31 (5), May

14 Machine specific dosimetry Daily check (pre-treatment QA) Prepare and check the scanning system before starting the daily patient treatments Takes place in the mornings Successful completion is a requirement for the patient irradiation Tasks can be finished in about 30 minutes Procedure consists of the following tests activities: Devices setup (Lasers, offsets of monitor chambers, air pressure etc.) Dosimetry check Beam energy and position checks Sweeper magnet and strip monitor test Range shifter check Safety interlock tests 14

15 Machine specific dosimetry Daily check phantom PMMA phantom Rotatable Alignment to various gantry angles Embedded ionisation chambers Two cylindrical Exradin chambers for absolute dose measurements One large parallel plate chamber (Ø 8 cm) for range measurement (range shifter scan) Scintillating foil (Lanex) Visual check of pencil beam 5 x 5 mini-strip chamber Phasespace measurement 15

16 Machine specific dosimetry Daily monitor calibration test IC 1 IC 2 6x6x6 cm 3 2Gy Measurement of the absolute dose in the center and at the distal falloff of a scanned cubic box of 6x6x6 cm 3 size. The dose is measured using a 0.5 cm 3 Exradin IC and a 0.05 cm 3 Exradin IC. Tolerance in dose: ±3% in the center of the box 16

17 Machine specific dosimetry Daily range measurement Range shifter scan: insertion of range shifter plates Ionisation current measured with large parallel plate chamber Ø8 cm collection volume 30 cm 3 Plotted against absorber material Dose This results in the well known Bragg peak curve Water equivalent depth 17

18 Machine specific dosimetry Daily range measurement (example for 177 MeV proton beam) Compare nominal range with measured one Check that the shape of the measured range curve coincides with the expected curve Tolerance in range: ±2 mm 18

19 Machine specific dosimetry (a) Dose Deviations: 0.7 % ± 0.4% 0.5 % ± 0.5% 0.8 % ± 0.6% (b) Range Deviations: 1.1 mm ± 0.3 mm 0.6 mm ± 0.4 mm 0.4 mm ± 0.4 mm Fig. 1: Results of the daily QA checks of absolute dose (a) and beam range (b). The measured doses for 138 MeV, 160 MeV and 177 MeV protons differ from the expected values by 0.7%, 0.5% and 0.8%, respectively, with standard deviations (SD) of 0.4%, 0.5% and 0.6%. The measured ranges differ from the nominal ranges by 1.1 mm, 0.6 mm and 0.4 mm with a SD of 0.3 mm, 0.4 mm and 0.4 mm. 19

20 Machine specific dosimetry Beam position and width measurements Check of parallelism and width of the pencil beam along the longitudinal beam axis (S-axis) 5 by 5 ministrip chamber similar to strip monitor mounted in the gantry nozzle The strip spacing: 4.5 mm (4 mm strip, 0.5 mm gap) Active area: 23 mm x 23 mm. The air gap between sensor and HVplate is 4 mm and the voltage used is 400 V. 20

21 Machine specific dosimetry Beam position and width measurement: S-Axis Scan pencil beam ± 15 cm pp IC thimble ICs mini-strip IC Tolerance in T and U direction: ±2 mm 21

22 Patient specific dose verification (IMPT) Patient 1 2 Phantom 4 3 Fields Spot weights Treatment plan 22

23 Patient specific dose verification (IMPT) Field 1 Field 2 Patient 1 2 Phantom 4 3 Fields Spot weights Treatment plan Field 4 Recalculated 3D dose distribution Field 3 23

24 Patient specific dose verification (IMPT) Dedicated water phantom Two ionisation chamber arrays 13 chambers each (0.1 ccm) Spacing: 1 cm Two orthogonal dose profiles Measures absolute dose in Gy Rotatable Alignment to gantry angle Adjustable water column Measurements at various depths Readout interface to planning system Online analysis of measured profiles against calculation 24

25 Patient specific dose verification (a) (b) Field verification for the first field of first patient treated with the COMET cyclotron Cervical cordoma case Correction of -1.0 mm in T-direction (table motion) and 2.0 % in dose Standard deviation: Gy 25

26 Patient specific dose verification mean: 0.8% σ: 1.3% mean: 0.2mm σ: 1.0mm mean: 0.0mm σ: 0.9mm mean: 0.3mm σ: 0.7mm Histograms of best fit parameters (based on 53 field measurements) No systematic positioning errors observed Small residual error of 0.8 % in total dose (still within our tolerance limits). 26

27 Patient specific dose verification 2D ionisation chamber arrays 2D-Array seven29 xdr, PTW Freiburg xdr = extended dose rate 27

28 CCD Dosimetry System Build at PSI (derived from M. Schippers at KVI) High spatial resolution: 0.5mm High reproducibility: 0.2% Fast data acquisition Suitable tool for relative dosimetry (patient specific & beam spot analysis) Lucite Plates Incident Field CCD Scintillating Lanex screen Mirror 28

29 CCD Dosimetry System Scintillation foil & CCD camera S.N. Boon et. al. Med. Phys Wedge-like dose distribution Varying monitor units linearily with coordinate Linear light yield (dose response) 29

30 CCD Dosimetry System for beam analysis Sweeper magnet scan [cm]: -9.0, -4.5, 0.0, 4.5, 9.0 Distances from nozzle [cm]: 7.0,14.5, 22.0, 29.5,37.0 Apply a Gaussfit Phase space (in air) for each energy and angle Sweeper Beam width Distance from nozzle Distance along beam axis 30

31 CCD Dosimetry System for field verification calculated dose difference dose-difference-histogram measured γ-evaluation γ-volume-histogram ΔD=3% Δd=3mm

32 CCD Dosimetry System Quenching Effects Under-response of scintillator in bragg peak region (high LET) Effect must be considered when dose measurements involve different parts of the bragg peak Inclusion of quenching in dose calculation by (empirical) correction factor: C = 1 / ( de/dx) Development of scinillating powder mixture without quenching 32

33 CCD Dosimetry System Magic mixture of two scintillating powders Correct quenching effects at the Bragg peak Gd 2 O 2 S:Tb (Zn,Cd)S:Ag 33

34 Radiochromic Films Protons PMMA High spatial resolution Self-developing no developer/fixer needed ideal for filmless radiotherapy departments Easy to handle at room light no dark room or light proof packaging No electronics required during measurement EBT Film Simultaneous measurements in various depths by stacking before Scan after Scan Needs postprocessing for quantitative analysis Sensitive to ultraviolet light, temperature Post-radiation colorization Under-response in bragg peak region (Quenching) 34

35 Radiochromic Films GafChromic EBT Film Usable dose range 0 8 Gy Near linear response in dose range < 2 Gy 35

36 Radiochromic Films Comparison of pre-calculated and actual beam entries for different combinations of gantry- and table angles end to end test of the treatment delivery system 36

37 Radiochromic Films End-to-end steering file check 37

38 Summary and Conclusions For absolute dosimetry use the standard protocols (IAEA TRS-398) Current protocols do not provide explicit recommendations for scanned beams Beam monitor calibration Dose verification Machine specific QA includes extensive checks on a daily basis Patient specific dosimetry is an essential part of the treatment Highly effective QA tools are needed QA protocol is needed to give advice to novice proton users 38

39 Thank you 39

40 40

41 Dosimetry and QA for proton radiotherapy 1. Absolute Dose Measurements for Protons 2. Machine Specific Checks and Dosimetry 3. Patient Specific Dosimetry 4. Summary 41

42 Faraday cup measurement Absolute dosimetry Monitor Protons unit per Absolute dose measurements Treatment planning: Dose directly calculation from based Bethe-Bloch on depth formulation dose curve 1,2 derived Beam flux Nuclear attenuation interaction (0,H cross-sections) effects 2 1,2 Dose homogenous phantom calculation in (10x10x10cm box) Thimble chamber measurements 1 Scheib S. Diss. ETH Zürich Nr Pedroni E. et al Phys Med Biol Feb 7;50:

43 The Scheib-Pedroni 1-d model Absolute dose measurements b) 2 Kinetic deposited energy in following from NI way: 33% at point of interaction 33% end approximation) of Distributed range (triangle linearly to 33% Lost (photons/neutrons) Arbitrary units a) e) f) c) d) 1 Beam width FWHM cm (f) a) Deposited dose b) Proton fluence c) Local deposited dose as result of NI d) Dose deposited by long range secondaries e) Dose from primary protons Depth in water (cm) Scheib 1993 (PhD thesis) 43

44 Monitor unit results for PSI Absolute dose measurements Energy Protons /MU Dose accuracy from model 138 MeV % 160 MeV % 177 MeV % 1 MU ~ 7000 Protons ~ 1fC (proton charge) 44

45 Absolute dose measurements The Scheib-Pedroni 1-d model In clinical practice Based measurements, too low on c.f. field calculation specific measured IC dose 1-9% Problem secondary of particles? lateral distribution of 45

46 The Pedroni extended model Absolute dose measurements ( P p NI NI (1 f ( w)) G ( σ ( w)) + f G ( σ ( ))) D( x, y, w) = T( w) w NI Models the lateral spread of long range secondary particles as a 2 nd gaussian in the dose calculation NI σ P (w) - Beam with of primary beam at depth w G P Gaussian distribution of primary beam σw NI (w) - Beam with of secondary particle distribution at depth G NI Gaussian distribution of secondary particle distribution secondary f NI (w) Fraction particles of total integral dose at depth w resulting from Pedroni et al, PMB, 50 (2005)

47 The Pedroni extended model Experimental determination of σ NI and f NI Frame experiment Absolute dose measurements 3cm 5cm 7cm A B 3cm C 5cm D 10cm 7cm Ionisation chamber Measure dose at centre of various frame sizes (A- D) Pedroni et al, PMB, 50 (2005)

48 The Pedroni extended model Absolute dose measurements Relative dose Frame A+B+C+D A Depth distributions frames dose (177 MeV) for From distributions, possible these σ NI and f NI to derive 10*B 10*C 10*D Depth in water (cm) Pedroni et al, PMB, 50 (2005)

49 The Pedroni extended model Experimental verification and clinical results Absolute dose measurements Dose (Gy) 10x10x10 cm field Global dose correction required due to NI effects over 390 measured fields 140 Depth in water (cm) Number of fields Dose (Gy) 3x3x10 cm field Without NI halo With NI halo Depth in water (cm) Dose correction (%) Pedroni et al, PMB, 50 (2005)

50 Daily range measurement Machine specific QA (a) Dose Deviations: 0.7 % ± 0.4% 0.5 % ± 0.5% 0.8 % ± 0.6% Range Deviations: 1.1 mm ± 0.3 mm 0.6 mm ± 0.4 mm 0.4 mm ± 0.4 mm (b) Fig. 1: Results of the daily QA checks of absolute dose (a) and beam range (b). The measured doses for 138 MeV, 160 MeV and 177 MeV protons differ from the expected values by 0.7%, 0.5% and 0.8%, respectively, with standard deviations (SD) of 0.4%, 0.5% and 0.6%. The measured ranges differ from the nominal ranges by 1.1 mm, 0.6 mm and 0.4 mm with a SD of 0.3 mm, 0.4 mm and 0.4 mm. 50

51 Challenges 1: Dosimetry Secondary particle effects in complex fields Without NI corrections With NI corrections Bolsi et al, SASRO,

52 Challenges 1: Dosimetry Secondary particle effects in complex fields IC Measurements in water for complex IMPT field With NI halo IC measurements With NI halo IC measurements Bolsi et al, SASRO,

53 Patient specific dose verifications 2D CCD dosimeter for field verifications Quenching Effects Under-response of scintillator in bragg peak region (high LET) Effect must be considered when dose measurements involve different parts of the bragg peak Inclusion of quenching in dose calculation by (empirical) correction factor: C = 1 / ( de/dx) 53

54 Summary and Conclusions For absolute dosimetry use the standard protocols (IAEA TRS- 398) Current protocols do not provide explicit recommendations for scanned beams Beam monitor calibration Dose verification Machine specific QA includes extensive checks on a daily basis Patient specific dosimetry is an essential part of the treatment Highly effective QA tools are needed QA protocol is needed to give advice to novice proton users 54

55 CCD Dosimetry System for field verification calculated measured 55

56 CCD Dosimetry System for field verification calculated measured 56

57 CCD Dosimetry System for field verification calculated measured 57

58 CCD Dosimetry System for field verification calculated measured 58

59 CCD Dosimetry System for field verification calculated measured 59

60 CCD Dosimetry System for field verification calculated measured 60